The experiments in this proposal have been designed to identify the cellular mechanisms underlying motor activity in the chick embryo. The experiments will be performed using an isolated spinal cord preparation that generates spontaneous motor activity. Intracellular recordings will be obtained from motoneurons to identify the nature of their synaptic drive during motor activity and the mechanism of alternation between flexor and extensor motoneurons. We will then determine how these cellular mechanisms become assembled during development. In younger embryos electrotonic recording and current passage through ventral roots will be used instead of intracellular methods. These techniques will allow us to determined when embryonic motoneurons acquire the ability to discharge and whether developmental changes in motoneuron firing are due to changes in synaptic drive, intrinsic motoneuron properties of some combination of both. Finally, we establish if the presence of the hindlimb is casually necessary for the development of motor activity by recording the pattern of ventral root discharge in embryos rendered limbless by surgery or by the limbless mutation. The experiments described in this proposal constitute the first cellular analysis of motor development in the chick embryo. They will not only provide new information about the development of vertebrate motor activity but also may provide insights into the neural mechanisms responsible for coordinated motor behavior in the adult nervous system.